1. Field
This invention relates to ceramics (powders or dense ceramic bodies) made by exothermic reactions and particularly to such ceramics which are fine-grained (i.e., submicron diameter particle sizes prior to densification).
2. Description of the Prior Art
Composites of titanium carbide (TiC) well dispersed in an Al.sub.2 O.sub.3 matrix are used as replacements for WC-Co cutting tools in finish machining operations. Because of their high hardness (excellent wear resistance) and good microstructure (high surface finish due to fine grain size and low porosity), these composites are also used as substrates for thin film transducers in the computer industry and in other wear applications. These composites are formed by mixing submicron Al.sub.2 O.sub.3 and TiC to form a homogeneous mixture and hot pressing the powder at temperatures near 1700.degree. C. Hot pressing limits the economical fabrication to simple parts because expensive diamond grinding is required for making complex components from hot pressed billets. In addition, the useful range of applications for Al.sub.2 O.sub.3 -TiC composites is limited due to their brittle nature. Methods for achieving higher toughness would therefore increase the usefulness of these composites.
Silicon carbide is a prime candidate for making high temperature components due to its excellent oxidation and creep resistance. Currently, silicon carbide (SiC) components are fabricated by sintering submicron SiC powder, with B and C (or B.sub.4 C) or Al and C (or Al, B, and C) as sintering aids. The highly reactive submicron particle size is needed to make the powders sinterable. Commercial routes for making submicron SiC rely upon either the pyrolysis of silane compounds, or the carbothermal reduction of silica, followed by grinding. Other ceramics (i.e., TiC, TiN, B.sub.4 C, TiB.sub.2, BN, TaC, WC, etc) are difficult to make as submicron powders but are useful for a variety of applications including wear parts, sputtering targets, electrodes, cutting tools, and armor. Clearly, a rapid, inexpensive method of making submicron powders would be an improvement in the art.
Various metals and alloys have been prepared for several decades by metallothermically reducing oxides. These reactions can be described in a general way as: EQU A+MO.fwdarw.AO+M (1)
wherein A is a metal, MO is a metal oxide, and AO and M are the metal oxide and metal, respectively, formed as a result of the exothermic reaction. The difference in density between the metal, M, and the metal oxide, AO, usually allow separation to occur when the heat generated by the reaction is high enough to melt one or both of the constituents. The synthesis of refractory cermets (ceramic-metal composites) and ceramic composites (porous mixtures of oxides, borides and carbides) by exothermic reactions was reported by Walton and Poulos (J. Am. Ceram. Soc., vol. 42, pp. 40-49, 1959). They showed that the heat generated by exothermic reactions to form cermets enabled a liquid phase to form and infiltrate the ceramic matrix.
The synthesis and densification of a wide range of ceramic and metallic materials by self-propagating, high temperature reactions has been reported in the Russian literature and has sparked renewed interest in exothermic reactions. Temperatures between 1500.degree.-3500.degree. C. have been measured during reactions involving a wide range of materials. These types of reactions are illustrated below: EQU M+X.fwdarw.MX (2)
wherein M is a metal and X is carbon, nitrogen, silicon, oxygen, or boron. This type of a reaction can be used to make powders as fine as 5 microns, although larger particle sizes generally result due to the high temperatures of the reaction. Pressure can be applied immediately after the reaction to densify ceramics which do not completely melt and cannot be cast. Previous researchers have not been able to synthesize submicron powders using the above technologies. In addition, there is no evidence in the literature that exothermic reactions have been used to product intimate mixtures of ceramics with microstructures which result in improved toughness. There is no evidence in the literature that pressureless sintering (densification accompanied by shrinkage) has been observed in any of the previous investigations. Since all of these features are desirable, the ability to make such ceramics would be an improvement in the art.